This invention relates to a binder system for use in the injection molding of metal powders, ceramic powders and cermet powders (these powders are hereunder collectively referred to as "sinterable powders"), as well as a molding compound containing the binder system.
Sinters are conventionally produced by a process that comprises mixing a sinterable powder with a binder, molding the mixture by a suitable technique such as casting, extrusion, jiggering or pressing, and sintering the molded part. However, it is difficult to produce sinters of complex forms by these molding methods. Under the circumstances, injection molding of sinterable powders is adopted as a method for producing more complexly molded parts.
Injection molding of sinterable powders is a technique commonly employed to mass produce small-sized sinters of complex shapes. The process starts with mixing a sinterable powder (raw material) with a binder to prepare a compound for injection molding. The compound is thermoplastic and can be molded into desired forms with an injection molding machine. This forming step is essentially the same as the molding of plastic materials and permits mass production. In the next step, the unwanted binder is removed from the molded part. This "debinding" step is performed either by heating the molded part so that the binder will evaporate or flow out or by holding the molded part in a solvent to extract the binder or by the combination of the two methods. A suitable debinding method is selected in accordance with the type of binder used. In the last step, the debound part is sintered to yield the final product.
Injection molding of sinterable powders is characterized by its ability to mold fine sinterable powders with which high sintered density can be attained. Fine powders having average particle sizes of no more than 10 .mu.m have heretofore been difficult to mold by pressing for several reasons such as poor flowability and the galling of molds. In contrast, even such fine powders can be easily molded by the injection molding method and, furthermore, complex three-dimensional molds can be produced. Because of these advantages, the injection molding of sinterable powders has recently gained popularity in the manufacture of pure iron parts, Fe-Ni base alloyed parts and stainless steel parts and its application is making way even to magnetic materials and cemented carbides.
Binders to be used in the injection molding of sinterable powders are roughly classified into two types, thermoplastic and thermosetting, but considering the recycling of sprues, runners, etc. unrecyclable thermosetting binders are not used very often. Commonly used thermoplastic binder systems are composed of such ingredients as thermoplastic resins, waxes, plasticizers and lubricants. The resins, which are the principal ingredient of the binder system, impart plasticity to the molding compound and insure the necessary strength for the molded part at room temperature. Furthermore, waxes and plasticizers which are organics having lower molecular weights than the resins are added to improve the moldability and debinding properties. Known resin components include polyethylene, polypropylene, polystyrene, ethylene-vinyl acetate copolymer (EVA), ethylene-ethyl acrylate copolymer (EEA), poly (alkyl methacrylate) and polyamides, and these may be used either alone or in admixtures. Components of lower molecular weights than the resins include paraffin waxes, higher fatty acids, higher alcohols, higher fatty acid esters, higher fatty acid amides, and phthalic acid esters such as diethyl phthalate and dibutyl phthalate, and these may used either alone or in admixtures. Thus, compounds as the feed stock for the injection molding of sinterable powders are typically in the forms of a mixture of sinterable powders and several organic additives including resins, waxes and plasticizers.
Sinterable powders such as metal powders and ceramic powders have near-hydrophilic surfaces, so they wet very poorly with hydrophobic binders which are chiefly composed of organics. Hence, it is difficult to have the sinterable powders dispersed uniformly in the binders and agglomerates will readily form. If the molding compound contains agglomerates which are not wetted with the binder, the compound becomes highly viscous and not only does this impair the moldability of the compound but also the strength of the green body drops so markedly that cracking is prone to occur. As a further problem, cracking and blistering are also prone to occur during the removal of the binder (debinding step).
Agglomerates are more prone to form as the molecular weight of the binder increases. Consider, for example polyethylene (PE), polypropylene (PP) and polystyrene (PS); these resins have long been used in the binder systems for powder injection molding but because of their low ability to wet sinterable powders, the molding compound has such low fluidity that it is difficult to be injection molded or the molded part has only low strength.
It is known that the wettability of sinterable powders with binders can be enhanced either by treating the surface of sinterable powders to render it hydrophobic or by introducing into binder components those functional groups which will wet the hydrophilic surface of the sinterable powders. The first approach can be implemented by treating the surface of sinterable powders with a silane-base coupling agent, a titanate-base coupling agent [Japanese Patent Publication (kokoku) SHO No. 59-41949] or an aluminum chelate compound [Japanese Patent Laid-Open Publication (kokai) SHO No. 61-242947]. The second approach can be implemented by using surfactants [Japanese Patent Laid-Open Publication (kokai) SHO Nos. 59-182267 and 59-35058] or by using binder components into which a carboxyl group, an ester, an amino group, a hydroxyl group, an acid anhydride and other functional groups that will interact with the surface of sinterable powders have been introduced, as exemplified by an ethylene-vinyl acetate copolymer (EVA) [Japanese Patent Laid-Open Publication (kokai) SHO Nos. 52-117909 and 58-135173], an ethylene-ethyl acrylate copolymer (EEA) [Japanese Patent Laid-Open Publication (kokai) SHO No. 59-121150], cyclopentadiene-unsaturated dicarboxylic acid anhydride copolymers or hydrogenation products thereof [Japanese Patent Laid-Open Publication (kokai) No. 62-12658], alpha-methylstyrene-unsaturated acid anhydride copolymers [Japanese Patent Laid-Open Publication (kokai) SHO NO. 63-252951], stearic acid [Japanese Patent Publication (Kokoku) SHO No. 36-7883], behenic acid [Japanese Patent Laid-Open Publication (kokai) HEI No. 2-267156], and polyalkylene carbonates [Japanese Patent Laid-Open (kokai) Publication HEI No. 1-257158 ]. These methods are effective in improving the wettability of sinterable powders with binders and the powders are dispersed so uniformly in the binders that the molding compound can be easily injection molded to produce satisfactory molded parts.
While the selection of a suitable binder is important for successful injection molding of sinterable powders, the conventional binder systems have had the following problems.
Since binder resins such as polyethylene and polypropylene only poorly wet the sinterable powders, it is difficult to disperse the powders completely and the mixture contains residual agglomerates, thus failing to produce a uniform molding compound having good fluidity. Even if a low-molecular weight component such as wax is added with a view to improving moldability, the added component is still low in miscibility with the sinterable powder and troubles such as separation between the binder and the powder have occurred during injection molding. As a further problem, if heat is applied to remove the binder in the debinding step, the molded part may occasionally deform or debinding defects such as blisters or cracks may occur during heating. In order to avoid these "debinding defects", the debinding time has to be extended and it takes an increased time to complete the step of binder removal.
The ethylene-vinyl acetate copolymer (EVA) and ethylene-ethyl acrylate copolymer (EEA) have polar functional groups and are highly miscible with sinterable powders; hence, these resins are extensively used as components of injection molding binder systems. Compounds using these resins as binder components have good moldability and the molded part also has high strength. However, in order for these resins to exhibit their performance fully, the content of vinyl acetate or ethyl acrylate must exceed a certain level but, then, the softening temperature of the binder will drop. As a result, the molded part is highly prone to deform in the debinding step, with occasional occurrence of blisters. Therefore, a prolonged time is also required for binder removal to give an immaculate and sound debound part.
Polystyrene and poly (alkyl methacrylate) esters have high tendency to depolymerize and, hence, they are easy to remove and are often used as binder components. However, these resins wet sinterable powders so poorly thai they are no more suitable for molding than polyethylene and polypropylene.
Under the circumstances, it has been proposed that two or three of the resins described above be used in combination in the injection molding compound but the proposed combinations have their own advantages and disadvantages and, as of today, no single binder system has been developed that is satisfactory in terms of both moldability and ease of debinding.
The method of treating the surface of sinterable powders in order to improve their wettability with binders has various defects such as the high cost of agents used for surface treatment, the addition of an extra step for surface treatment, and the tendency of Ti, Si, and Al used as surface treating agents to remain in the sinter, thereby causing adverse effects on its properties. The addition of surfactants or higher aliphatic acids causes a drop in the softening temperature of the binder and, hence, defects such as deformation and blistering will readily take place upon application of heat for removing the binder. The proposal made by Japanese Patent Laid-Open Publication (kokai) SHO No. 62-12658 uses a polymer containing unsaturated bonds, so gelation is prone to occur during mixing and the molding compound becomes highly viscous or difficulty is encountered in the effort to recycle sprues or runners. The alpha-methylstyrene-unsaturated acid anhydride copolymer used in Japanese Patent Laid-Open Publication (kokai) SHO 63-252951 has such a high softening temperature and viscosity that it is also necessary to use large amounts of solvents such as toluene and alcohol and, as a result, not only does a significant change in density occur on account of the evaporation of solvent but there also occurs a marked drop in strength, thereby presenting difficulty in accomplishing injection molding. Furthermore, whichever method is used, prolonged application of heat is necessary in order no remove the binder without causing deformation and if the debinding time is shortened, blistering or deformation will inevitably occur.